Vector versus scalar beams in atmospheric turbulence: scintillation index and average received intensity

The performance of free-space optical systems is impaired by atmospheric turbulence. Recently, vector beams have been considered more robust against atmospheric turbulence than scalar beams because their orthogonally polarized components can exhibit partially uncorrelated fluctuations. However, it remains unclear whether this advantage is fundamental or depends on the specific scalar reference used for comparison. In this work, we compare arbitrary vector beams with their constructed scalar counterparts in weak atmospheric turbulence through the pointwise average received intensity and scintillation index under the same transmit-power constraint. For any arbitrary vector beam having a spatially nonuniform polarization distribution, we theoretically construct a unit-power scalar counterpart that reproduces the same pointwise scintillation index within the first-order Rytov approximation. We further show that the scalar counterpart provides a higher pointwise average received intensity under weak atmospheric turbulence. Numerical simulations over kilometer-scale propagation and experimental measurements with full Poincaré beams corroborate these theoretical predictions.